The invention relates to a method for carbonitriding at least one component in a treatment chamber, in which at least one process gas is introduced into the treatment chamber as well as to a treatment chamber and an open-loop and/or closed loop control device for such a treatment chamber.
Methods for carbonitriding metal components are known from the German patent publications DE 199 09 694 A1, DE 101 18 494 A1 and DE 103 22 255 A1. Carbonitriding of metal components is a thermochemical process, in which carbon and nitrogen are introduced into the surface layer of an iron-based material. It is a particular kind of “case hardening”. The German patent publication DE 199 09 694 A1 describes a carbonitriding method, in which the diffusion of nitrogen occurs during the entire process or when using nitrogen as the donating gas preferably in the last process phase. The German patent publication DE 101 18 494 C2 describes a low-pressure carbonitriding method, in which steel components are initially carburized and subsequently nitrided with a nitrogen-donating gas. The German patent publication DE 103 22 255 A1 describes a method for carburizing steel components, in which nitrogen producing gas is added during the heating-up phase as well as during the diffusion phase. The nitrogen diffused during carbonitriding leads to an improved resistance to wear due to friction and to an improved resistance to tempering in the surface layer.
The process control during carbonitriding takes place in at least one treatment chamber by the presetting of pressure, temperature, time, process gas composition and process gas flow volume. During carbonitriding, molecular hydrogen can develop as a by-product from the carbon-donating and nitrogen-donating gases. The hydrogen content can be detected by suitable sensors. The sensors being used must be designed for use in low-pressure or vacuum systems.
In conventional gas-nitriding processes, commercially available hydrogen sensors allow for the control of the process gas atmosphere with the aid of the nitriding index. The nitriding index is defined as follows:kN=pNH3(pH2)1.5, wherein
kN=Nitriding index
PNH3=Pressure of the ammonia, and
PH2=Pressure of the hydrogen,
and describes the relationship between ammonia supply and ammonia conversion and consequently determines the excess supply of ammonia. In the case of gas-nitiriding processes exceeding the processing time, constant, reproducible nitriding conditions can be set independently of the size of the surface of the component charge by means of controlling in accordance with the nitriding index.
In the case of low-pressure carbonitriding processes, a control in accordance with the nitriding index is not possible because the carbon and nitrogen concentration and thereby the carbon and nitrogen absorption constantly change on the component surface while the process is being carried out, whereby no constant, reproducible carburizing and nitriding conditions can be set when the nitriding index is held constant. The progression of the gas decomposition or respectively of the resulting reactions occurs as a function of pressure, temperature and as a function of the reactive or catalytically active surface of the component charge or furnace lining.
The residence time of the gases in the treatment chamber resulting from the rate of flow is therefore crucial for the atmospheric composition in said treatment chamber.
For this reason, solid gas quantities for carrying out the process are in practice empirically ascertained through an elaborate series of tests, said solid gas quantities however only apply to the tested charge structure, the treatment chamber and the material of the metal components which is used. A transfer of the solid gas quantities to other process implementations, materials, charge structures and treatment chambers is not directly possible. These would have to again be empirically ascertained. The German patent publication DE 101 18 494 C2 describes a low-pressure carbonitriding using solid gases.